1. Technical Field
The present invention relates in general to electrical circuits, and in particular, to time-based control of a system having an integration response.
2. Description of the Related Art
FIG. 1 is a high level schematic diagram of a prior art integrator circuit 100 including a switch 102, an integrator 104, and control logic 106. As shown, in response to a control signal 108 generated by control logic 106, switch 102 connects the input of integrator 104 either to an input A that causes integrator 104 to integrate up or to an input B that causes integrator 104 to integrate down. Control logic 106 generates control signal 108 based upon the output of integrator 104 and a target signal 110 in order to cause the average value of the output of integrator 104 to match target signal 110. In practice, integrator 104 often is not an ideal integrator having infinite DC gain, but is instead a “leaky” integrator having a finite DC gain.
Integrator circuit 100 of FIG. 1 can be realized in a variety of different applications. For example, one application of integrator circuit 100 of FIG. 1 is the prior art boost-mode switching regulator 200 illustrated in FIG. 2A. Boost-mode switching regulator 200 includes an inductor 204 of inductance L connected to a diode 212, and a capacitor 218 connected between diode 212 and a voltage rail 214. A DC voltage Vx exists between the input of inductor 204 and voltage rail 214, and a DC voltage Vcap characterizes capacitor 218. Boost-mode switching regulator 200 farther includes a switch 202 connected between voltage rail 214 and the common node 216 of inductor 204 and diode 212. In the depicted implementation, switch 202 intermittently shorts common node 216 to voltage rail 212 in response to a control signal 208 generated by control logic 206 based upon a sensed current signal 220 (i.e., the switch current) and a target current signal 210. Although in the depicted implementation, the sensed current is the switch current, in other common implementations the inductor or diode current is sensed. In still other alternative implementations, control logic 206 can control switch 202 based upon the sensed voltage (e.g., output voltage Vcap) and a target voltage signal.
During the shorting interval when switch 202 is closed, the current through inductor 204 integrates linearly at Vx/L amperes per second, assuming ideal components (e.g., diode, inductor and switch). When switch 202 is opened, the voltage reverses, and the current through inductor 204 decreases at a rate of (Vcap−Vx)/L amperes per second, again assuming ideal components. In this open state, the voltage across inductor 204 adds with the input voltage Vx present between the inductor input and voltage rail 214 to produce a voltage greater than Vx. Thus, boost-mode switching regulator 200 increases or “boosts” the input voltage Vx by an amount governed by the duty cycle of switch 202.
A second application of the integrator circuit of FIG. 1 is the prior art buck-mode switching regulator 250 depicted in FIG. 2B. Buck-mode switch regulator 250 includes a switch 252 connected to an inductor 254 of inductance L, and a capacitor 268 connected between inductor 254 and a voltage rail 264. A DC voltage Vx exists between the input of switch 252 and voltage rail 264, and a DC voltage Vcap that ranges between 0 V and Vx characterizes capacitor 268. As depicted, buck-mode switching regulator 250 further includes a diode 262 connected between voltage rail 264 and the common node 216 of inductor 254 and switch 252. Switch 252 opens and closes in response to a control signal 258 generated by control logic 256 based on a sensed current signal 260 and a target current signal 262. As noted above with respect to boost-mode switching regulator 200 of FIG. 2A, in alternative implementations control logic 256 of buck-mode switching regulator 250 may control switch 252 based upon the inductor or diode current, or alternatively, based upon a sensed voltage (e.g., output voltage Vcap) and a target voltage signal.
When switch 252 is closed, the voltage across the inductor is equal to Vx−Vcap, and the current through inductor 254 integrates up linearly at Vx/L amperes per second, assuming ideal components. No current flows through diode 262 due to the reverse-bias of Vx. When switch 252 is opened, diode 262 is forward biased, the voltage across inductor 254 is equal to −Vcap (neglecting diode drop), and the current through inductor 254 integrates down linearly at a rate of (Vcap−Vx)/L amperes per second, again assuming ideal components. Thus, buck-mode switching regulator 200 decreases or steps down the input voltage Vx by an amount governed by the duty cycle of switch 252.
Prior art designs of integrating circuits, such as switching regulators 200 and 250 of FIGS. 2A-2B, are characterized by control logic (e.g., control logic 106, 206 or 256) that varies the duty cycle of the switch based upon the relative magnitudes of the target signal and a sensed current or voltage signal.
For example, in one conventional feedback control methodology, the duty cycle of the switch in an integrator circuit is controlled by a control signal produced by comparing the magnitude of a target voltage signal with a reference saw-tooth signal to achieve a duty cycle proportional to the difference between the target voltage signal and the output voltage Vcap. In one conventional current-mode control methodology, the control logic turns on the switch in response to a constant frequency clock pulse and turns off the switch when the sensed switch current is equal in magnitude to the target current. Both of these conventional control techniques are vulnerable to input voltage transients (e.g., from unregulated input voltage sources) and require at least several cycles to dampen the consequent oscillations in the output voltage. If current-mode control is employed, stabilizing the system may require additional substitution of an artificial downward sloping ramp for the sensed current signal.
A third control methodology known as One Cycle Control has been developed for applications having a constant switching frequency. In One Cycle Control, the control logic uses a constant frequency clock pulse to turn on the switch and uses an integrator to integrate a sensed voltage (e.g., the voltage of node 266 of buck-mode switching regulator 250). The control logic turns off the switch when a comparator indicates the integrated sensed voltage is equal to the target voltage signal. While One Cycle Control provides improved response to input voltage transients as compared to other control methodologies, the control logic required to implement One Cycle Control is unduly complex.
In view of the foregoing, the present invention appreciates that improved control for an integrating system would be useful and desirable.